The first case: as a company, if it has no influence, it can completely ignore the behavior of employees, because there is no need to care about what others say is the freedom of others. As a company, it has no right to interfere with the freedom of speech of others.
In the second case, if the employee's behavior has a bad influence on the company, you can send someone to negotiate with the employee and ask him to stop speaking ill, otherwise he will be investigated for libel and tort liability.
The third situation: if the employee's bad words have caused serious influence and consequences to the company, you can try to collect evidence. When enough evidence is collected, you can pursue the employee's responsibility through legal channels and ask him to bear the tort liability.